[webpack5] Complile for ES5 (#15708)

I think this is necessary for IE11.

via [Webpack docs](https://webpack.js.org/migrate/5/#turn-off-es2015-syntax-in-runtime-code-if-necessary)


> By default, webpack's runtime code uses ES2015 syntax to build smaller bundles. If your build targets environments that don't support this syntax (like IE11), you'll need to set output.ecmaVersion: 5 to revert to ES5 syntax.


Thank you

packages/next/build/webpack-config.ts

@@ -710,6 +710,7 @@ export default async function getBaseWebpackConfig(
       ignored: ['**/.git/**', '**/node_modules/**', '**/.next/**'],
     },
     output: {
+      ...(isWebpack5 ? { ecmaVersion: 5 } : {}),
       path: outputPath,
       // On the server we don't use the chunkhash
       filename: isServer